1. Field of the Invention
The present invention relates to methods and apparatus for a next-in-line protecting device to periodically broadcast its existence in a communication system.
2. Description of the Related Art
The present invention incorporates by reference the Institute of Electrical and Electronics Engineers (IEEE) 802.22.1 Standard for Information technology: “Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements, Part 22.1: Enhanced Protection for Low-Power, Licensed Devices Operating in Television Broadcast Bands”. 
IEEE 802.22.1 defines a beacon network including beaconing devices that offer enhanced protection for low power, licensed devices, such as wireless microphones, operating in television broadcast bands. The beacon network is constructed with a plurality of protecting devices. A protecting device (PD) is a beaconing device that is protecting a low-power, licensed device. The protecting device may be either the primary protecting device (PPD) or a secondary protecting device (SPD). A primary protecting device (PPD) is a device that uses periodic beacons to protect its corresponding licensed device, and its protection may be extended to other licensed devices in the area. A secondary protecting device (SPD) is a device that shares the responsibility of protecting its corresponding licensed device with the PPD. An SPD occasionally sends beacons for communicating with the PPD. A next-in-line protecting device (NPD) is an SPD that will be the first candidate to become a PPD in the event that the already existing PPD ceases operation. For the convenience of description, a cluster is defined as one PPD together with all the SPDs and the NPD whose information has been aggregated, or to be aggregated into the PPD's beacon frame.
In the IEEE 802.22.1 drafts v1.0 and v2.0, a method is defined for the NPD to periodically broadcast its existence. The NPD shall transmit the NPD codeword at least once every ten superframes (a constant parameter called aNPDPeriod). A MLME-NPD-LOST.indication primitive is generated by a medium access control (MAC) sublayer management entity (MLME) of the PPD or an SPD in the cluster and issued to its next higher layer as a notification that the MLME has not received either an NPD beacon frame or the NPD codeword within the last fifty superframes. In this way, the next higher layer is informed of the abnormal operation of the NPD so that the PPD and the SPDs could take proper actions.
In the current IEEE802.22.1 draft, whenever an SPD or the NPD wants to transmit a beacon frame to the PPD, the SPD or the NPD should first transmit a request-to-send (RTS) codeword at the receiving period of a superframe. If the SPD or the NPD receives the corresponding acknowledgement message (ACK) from the PPD, the SPD or the NPD will transmit the beacon frame at the next superframe. The SPDs and the NPD, however, shall avoid meaningless contention. Namely, the SPDs and the NPD shall not contend to send RTS codewords if the PPD is not transmitting its beacon frame in the current superframe. Otherwise, it is guaranteed that the PPD will ignore any request from the NPD and SPDs. This is because the PPD has to transmit its beacon frame at least once every other superframe, thus, the PPD cannot honor any contention.
In contemporary transmissions, it has long been the inviolate custom followed in order to preserve the allocation of time and frequency resources, for the next-in-line protecting device to wait until its internal timer expires, and only after the expiration of its internal timer, does the next-in-line protecting device transmit the NPD codeword. I have discovered that this contemporary transmission practice is not an efficient use of the allocation of time and frequency resources.
In other words, contemporary practice requires that the NPD shall not transmit when the PPD has not sent NACK in the last superframe. The disadvantage of the contemporary practice is that the periodic transmission of the NPD codeword by the NPD increases the probability of collision between the transmissions from the NPD and the SPDs. The collision events could result in many undesirable consequences. Firstly, the SPDs and the PPD are likely to conclude by mistake that the NPD has disappeared if all of the five attempts of the NPD transmission have failed in the last 50 superframes. Secondly, the collision wastes the opportunity for an SPD to transmit its beacon information timely. Thirdly, the collision may prohibit an SPD from sending a beacon frame timely, which in turn could make the PPD to believe the SPD has disappeared abnormally.